Constitution and properties of silica-alumina-catalysts

Since Eugene Houdry discovered some clays to be excellent catalysts for the “cracking” of heavy oils, the surface properties of silica-alumina combinations have been studied in various places and from various angles. The surface properties of silica-alumina-combinations differ in many ways from those of silica or of alumina, as such. Three different procedures for preparing the silica-aluminas may be mentioned, viz., (a) adsorption (chemisorption) of aluminium hydroxide on a wet silica surface, (b) precipitation of aluminium hydroxide on a wet silica gel, (c) co-gelling of a silica acid-and an aluminium salt solution. The influence of the alumina adsorption on the specific surface area, as measured with the B.E.T.-method and with the lauric acid method will be discussed. An attempt is made to give a picture of the binding of aluminium hydroxide on the silica, which leads to three possibilities, depending on the relative amount of both constituents. One of these possibilities leads to preparations with an acid character.